A machine for processing elements in the form of sheets, for example sheets of paper or of cardboard for the manufacture of packagings, comprises a succession of workstations for converting the sheets, in particular by means of a platen press. Since this type of machine is automated, conveying means are provided for bringing each sheet successively into each of the workstations. In practice, this usually comprises a series of transverse bars provided with grippers. These transverse bars, commonly referred to as gripper bars, each in turn grasp a sheet at its front edge before pulling it successively into the various workstations of the machine. Each end of each gripper bar is connected respectively to a lateral chain. The chain is formed into a loop, commonly referred to as a chain set. By virtue of a movement transmitted to the chain sets, all of the gripper bars start from a stopped position, accelerate, reach a maximum speed, decelerate and then stop. This describes an acceleration and speed cycle corresponding to the transfer of a sheet from one workstation to the following workstation. Each station performs its work in synchronism with this cycle, which is commonly referred to as the machine cycle.
Vibration phenomena occur along the chain sets and disturb their dynamic behavior. In order to limit these vibration phenomena, springs are used to serve as tensioners for the chain sets. The spring applies a force to a guide device in contact with the chain. It is possible in particular for the guide device to comprise a pulley, toothed wheels or simple guides.
Due to increases in machine production rates, the gripper bars and the chain sets are subjected to increasingly high mechanical stresses and therefore have to be increasingly strong. The vibrational phenomena are also more violent, and require the use of increasingly powerful springs which exert greater tensions. This further increases the mechanical stresses on the chain sets, thereby reducing their use life and increasing the risks of breakdown or breakage. It then becomes necessary to reinforce the chain sets, and therefore increase the power of the driving and braking elements which will accelerate them and decelerate them during each cycle.
Document WO2012/038035 describes an improved chain tensioner comprising at least one driving member able to generate a variable force with an intensity that is a function of the instantaneous production rate of the machine, the variable force being applied to at least one of the chain-guiding devices. This dynamic tensioner thus adapts the force applied to the guiding device permanently as a function of the machine angle and the production rate of the machine. As vibrations along the chain sets increase as a function of the production rate of the machine, the tension can therefore be adjusted accordingly. A variable thrust force in the chain sets as a function of the machine angle and the instantaneous production rate of the machine makes it possible to effectively combat the vibrational phenomena while limiting the stresses on the chain sets. This makes it possible to greatly slow down their wear and substantially increase their service life.